Phase transitions in lipid bilayers and biomembranes will be studied by the method of equilibrium density measurements. A more differential volumetric apparatus has been built and is now being tested. This device allows measurements to be made on smaller samples while being stirred, and has enough sensitivity to investigate the lower transition in the phosphatidyl cholines. A fundamental point to be investigated is the order of the main transition of the phosphatidyl cholines and also the phosphatidylethanolamines, and other lipids. The effects on both transitions of adding third components to the various lipids in water will be investigated.